This application is based on Patent Application No. 2000-209101 filed Jul. 10, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a substrate for an ink jet print head, an ink jet print head and a manufacture method thereof, and more particularly to a structure of a bump electrode pad used for electrical connection between the substrate and electric wiring such as a TAB tape, both forming the ink jet print head.
2. Description of the Prior Art
The manufacture of an ink jet print head involves a process of connecting two components: a head chip (hereinafter simply referred to as a xe2x80x9cchipxe2x80x9d or xe2x80x9cprint element substratexe2x80x9d), which is composed of a substrate has formed therein heaters and a driver IC and matrix wires for driving the heaters and a nozzle forming member in which ink ejection ports are formed, and a TAB tape electrically connects the print head to a printer body. This connecting process is normally performed by using both heat and ultrasonic wave to connect the bumps provided on the electrode pad on the chip to inner leads of the TAB tape.
A commonly used bump is a so-called plated bump which is formed by forming and patterning a SiO2 or SiN film as a passivation layer, forming one to three layers of barrier metal such as Ti, Cu and W as a contact improving layer on aluminum electrodes, forming a resist pattern over the barrier metal layer by photolithography, and finally growing gold by electrolytic plating.
The forming of the plated bump requires performing several cycles of a vacuum film forming process and an exposure/development process. Because in the case of the plated bump an entire wafer is subjected to the plating process, not only sound chips but also chips that become faulty in the subsequent steps are formed with gold-plated bumps. This leads to a possible increase in cost. Further, when the number of electrode pads per the wafer is small (i.e., when the number of bumps is small), the cost per bump increases.
For these reasons, an increasing number of a ball bumps are being used in recent years. The ball bump is formed by using the wire bonding method. In this forming process an arc discharge is applied to the front end of a wire passed through a ceramic tube called capillary to form a ball, which is then joined to a predetermined electrode pad on the substrate by using both heat and ultrasonic wave. Then, the capillary is lifted while at the same time the wire is held by a cut clamper, thus fracturing the wire by a tensile strength to cut off the ball portion and thereby form a bump. Another method of joining the balls to the electrode pad is known to use only heat, rather than using both heat and ultrasonic wave as in the above method.
As described above, the ball bump does not require the expensive vacuum film forming device and exposure device as do the plated bump. Further, because the passivation film and the barrier metal are not necessary, the ball bump is more advantageous than the plated bump in terms of cost when the number of pads per a piece of wafer is small.
In an ink jet print head that uses thermal energy produced by a heater to eject ink, films making up the heater or the like tend to decrease in thickness. The structure of this type of print head will be discussed as follows in terms of the thickness of the film tending to decrease.
An example substrate making such an ink jet print head is made by successively forming on a silicon base an IC film (made up of about six layers) for a driver IC or the like which consists of semiconductor devices to drive the heater in ejecting ink, a first interlayer insulating film (e.g., SiN film) forming a lowermost layer in contact with the base, a first electric wiring film (e.g., Al film) forming a common electrode for supplying an electric power to drive the heater by the driver IC in response to a drive signal or a common electrode for grounding, a second interlayer insulating film (e.g., SiO film) overlying the first electric wiring film, a heater film (e.g., TaN film) forming the heater, a second electric wiring film (e.g., Al film) directly connected to the heater to supply an electric power to the heater, and a wear resistant film (e.g., Ta film) overlying the second electric wiring film.
FIG. 16 is a plan view showing a conventional example of a heater and an electric wire for driving the heater corresponding to one ejection port in the substrate for the ink jet print head of the type described above. FIG. 17 is a perspective view showing a head chip made by forming, on the substrate having the electric wiring film or the like, a nozzle forming member in which ink ejection ports or the like are formed.
In order to selectively drive a plurality of heaters to eject ink according to print data, the substrate for the print head is normally formed with a matrix electrode wire. In FIG. 16 a first electric wire 202 represents a common electrode forming a part of the matrix wire and is connected in a through-hole portion 105 to a second electric wire 205 which in turn is connected to a heater film 204 forming a heater 101. More specifically, as described later by referring to FIG. 18, the first electric wire 202 is formed as lower layer with respect to a direction of thickness of the substrate, and this wire 202 and the second electric wire 205 formed as an upper layer than the wire 202 are generally formed in separate steps in a substrate making process and thus are electrically interconnected via the through-holes. Further, as to the connections for supplying an electric power and a drive signal to the head chip and connections for grounding the substrate potential, the substrate is formed at its end portions with electrode pads 110, as shown in FIG. 17, for electrical connection to a printer body.
FIG. 18 is a cross section showing a film structure of mainly the heater portion 101, the through-hole portion 105 and the electrode pad portions 110 in the above substrate structure.
The film structure of the heater 101 and its vicinity is presented in FIG. 18 as a cross section taken along the line 18axe2x80x9418a of FIG. 16. On the silicon base 11 are laminated a first interlayer insulating film 201, a second interlayer insulating film 203, a heater film 204, a part of the second electric wire film 205, a protective film 206, and a wear resistant film 207.
In FIG. 18 the film structure of the through-hole portion 105 that connects the first electric wire film 202 and the second electric wire film 205 is presented as a cross section taken along the line 18bxe2x80x9418b of FIG. 16. On the silicon base 11 are successively laminated the first interlayer insulating film 201, the first electric wire film 202, the second interlayer insulating film 203, the heater film 204, the second electric wire film 205, the protective film 206 and the wear resistant film 207. In this film structure, the second interlayer insulating film 203 is partly formed with through-holes to electrically connect the first electric wire film 202 to the second electric wire film 205 through the heater film 204.
Further, in FIG. 18 the film structure of the electrode pad is presented as a cross section taken along the line 18cxe2x80x9418c of FIG. 17. The first interlayer insulating film 201, the first electric wire film 202, the heater film 204, and the second electric wire film 205 are successively laminated.
As described above, although the first electric wire film 202 and the second electric wire film 205 are electrically connected together, they are formed as separate films owing to different functions performed. That is, they are formed in separate manufacture processing steps. In more concrete terms, the first electric wire film 202 is formed under the heater film 204. On the other hand, the second electric wire film 205 is formed over the heater film. For the sake of the film making process, the heater film 204 and the second electric wire film 205 are also formed in the electrode pad portion 110 along with the heater portion 101 and the through-hole portion 105. The second electric wire film 205 in the electrode pad portion 110 forms a surface conductive film in contact with the ball bumps.
In the bubble jet type print head composed of the substrate with the above-described structure, the density of ink ejection ports and their associated structures in the print head are being increasingly enhanced in recent years to cope with the growing demands for faster printing and higher print quality. Such an increase in density may cause a problem of a heat generation or heat storage. For example, the heat generated by the heater in ejecting ink is mostly released outside together with the ejected ink droplet, with the remaining heat, which is small, accumulated in the head when the printing process continues. When the ink ejection ports are arranged in high density, the extent to which the heat is accumulated increases, causing the head temperature to rise, resulting in an ejection failure or a broken head.
To deal with this problem, it is important to minimize the amount of energy applied to the print head for ink ejection. In this respect, measures to improve the thermal efficiency of ink ejection include, for example, reducing the thickness of the protective film over the heater film to transfer heat to the ink with an increased efficiency. For example, reducing the thickness of the protective film from the conventional 8000 xc3x85 to 3000 xc3x85 can reduce the energy applied to the print head at time of ink ejection by about 40%.
Such a reduction in the thickness of the protective film, however, degrades a coverage by the protective film of stepped portions of the electric wires. To deal with this situation, the second electric wire film 205 such shown in FIG. 18, which is covered by the protective film and formed over the heater film, is reduced in thickness to minimize a vertical difference between levels at the stepped portion and thereby prevent the deterioration of step coverage. For example, the aluminum film of the electric wire is reduced in thickness from the conventional 4,000 xc3x85 to 2,000 xc3x85.
However, the above-described reducing the thickness of the second electric wire causes reducing the thickness of the second electric wire film in the electrode pad portion, i.e., the surface conductive film in contact with the ball bumps. As a result, the ball bumps may result in a faulty joint and, in the worst case, may cause a bump loss, the phenomenon in which bumps come off the electrode pad. For example, when gold is used as a material of the ball bump and an aluminum electric wiring layer is used as a surface conductive film that comes into contact with the bumps on the electrode pad, the frequency of the bump loss generally increases as the thickness of the aluminum electric wiring layer decreases.
As described above, a trouble may occur in which the surface conductive film fails to adhere to the ball bumps or their joining strength is weak (generally evaluated by the strength measured by a shear tester). This is explained as follows. Because the second electric wire film of, for example, aluminum formed over the relatively hard heater film is thin, resulting in a reduced joining strength of an alloy of gold ball bump and aluminum joined by ultrasonic bonding. Increasing the intensity of the ultrasonic wave for solving this problem, however, may cause cracks in the pad portion in the substrate. Further, to minimize the energy necessary for ink ejection requires a further reduction in the thickness of the protective film and its associated second electric wire film, for example, down to 1,500 xc3x85 and 1,000 xc3x85, respectively. This in turn makes the problem of poor junction of ball bumps more significant.
An object of the present invention is to provide a substrate for an ink jet print head, an ink jet print head and a method of manufacture thereof, which assures a satisfactory joint between a ball bump and an electrode pad regardless of a reduction in the film thickness in the substrate for the ink jet print head.
In a first aspect of the present invention, there is provided a substrate for an ink jet print head that uses thermal energy to eject ink, the substrate comprising:
a film structure having a plurality of films laminated over the substrate, the plurality of films including a first electric wire film, a heater film and a second electric wire film formed one upon the other in that order over the substrate, a combination of the heater film and the second electric wire film allowing the thermal energy to be generated;
wherein an electrode pad portion, which is formed at a part of the film structure to make electrical connection to other than the substrate through a ball bump, is formed by an exposed part of the first electric wire film, and a thickness of the first electric wire film is larger than that of the second electric wire film.
In a second aspect of the present invention, there is provided an ink jet print head which uses thermal energy to eject ink, comprising:
a substrate making the ink jet print head, the substrate including:
a film structure having a plurality of films laminated over the substrate, the plurality of films including a first electric wire film, a heater film and a second electric wire film formed one upon the other in that order over the substrate, a combination of the heater film and the second electric wire film allowing the thermal energy to be generated;
wherein an electrode pad portion, which is formed at a part of the film structure to make electrical connection to other than the substrate through a ball bump, is formed by an exposed part of the first electric wire film, and a thickness of the first electric wire film is larger than that of the second electric wire film.
In a third aspect of the present invention, there is provided a method of manufacturing an ink jet print head which uses thermal energy to eject ink, the method comprising the steps of:
forming a substrate, the substrate constituting the ink jet print head and having a film structure, the film structure having at least a first electric wire film, an interlayer insulating film, a heater film, a second electric wire film and a protective film laminated one upon the other in that order over the substrate, a combination of the heater film and the second electric wire film allowing the thermal energy to be generated;
wherein, in an electrode pad portion formed by a part of the step of forming the film structure of the substrate and adapted to make electrical connection to other than the substrate through a ball bump, the interlayer insulating film is removed to expose the first electric wire film and to make an exposed part of the surface first electric wire film be a part to which the ball bump are joined.
In a fourth aspect of the present invention, there is provided a method of manufacturing an ink jet print head which uses thermal energy to eject ink, the method comprising the steps of:
forming a substrate, the substrate constituting the ink jet print head and having a film structure, the film structure having at least a first electric wire film, an interlayer insulating film, a heater film, a second electric wire film and a protective film laminated one upon the other in that order over the substrate, a combination of the heater film and the second electric wire film allowing the thermal energy to be generated;
wherein, in an electrode pad portion formed by a part of the step of forming the film structure of the substrate and adapted to make electrical connection to other than the substrate through a ball bump, after the interlayer insulating film is patterned to form a window therein, the heater film and the second electric wire film are deposited one upon the other and then removed to expose the first electric wire film and to make an exposed part of the surface first electric wire film be a part to which the ball bump are joined.
With the above construction, because the exposed part of the first wire electrode underlying the heater forms the electrode pad, a film which does not need to be reduced in thickness to secure the heater protective film""s step coverage even when the protective film is made thinner can be used for the electrode pad. Further, an inherently thick film can be used for the electrode pad. As a result, when the ball bump is joined by an ultrasonic bonding process bonding, the bonding strength can be increased.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.